This invention relates to a superconducting filter apparatus used at a base station in mobile communications, and to a wireless receiving amplifier having a superconducting filter. More particularly, the invention relates to a superconducting filter apparatus that is capable of rapidly detecting an abnormality in a refrigerator, and a wireless receiving amplifier having a superconducting filter.
Generally, in order to obtain a steep cut-off characteristic in a communications filter, the number of filter stages must be increased. However, a problem which arises is a commensurate increase in loss in the pass band. Accordingly, note has been taken of the fact that a superconductor has a resistance that is lower than that of ordinary metals by two to three orders of magnitude, and a superconducting filter that holds loss in the pass band to the minimum has been put into practical use employing a superconductor as the conductor of the filter. Such a superconducting filter has become the focus of much attention in recent years for the purpose of effectively utilizing frequency in mobile communications, increasing subscriber capacity and increasing base-station coverage area, etc. A known example of a superconducting material for a superconducting filter is YBCO (Y—Ba—Cu—O), which has a critical temperature (Tc) on the order of 90 K. This material is used at a temperature Tc on the order of 70 K, which is a temperature at which superconducting characteristics are stable.
FIG. 18 is a diagram showing the structure of a conventional wireless receiving amplifier having a superconducting filter. A superconducting filter (SCF) 1 and low-noise amplifier (LNA) 2 are secured to a cold head 4 and accommodated within a vacuum vessel 3. The cold head 4 is cooled by a refrigerator 5. The superconducting filter 1 and low-noise amplifier 2 are cooled by the refrigerator 5 via the cold head 4 and operate at Tc=70 K. The vacuum vessel 3 and refrigerator 5 are disposed in a case 6 in such a manner that outdoor installation is possible. Terminals 7a, 7b and 8a, 8b provided on the case 6 and vacuum vessel 3 are connected by coaxial cables 9a, 9b, respectively, and terminal 7b, superconducting filter 1, low-noise amplifier 2 and terminal 8b are connected by a coaxial cable 9c. 
As shown in FIGS. 19(A), 19(B), the superconducting filter 1 has a structure in which a filter electrode 1b (see FIG. 19(A)) and an n-stage (n=5 in the illustration) λ/2 resonator 1c (see FIG. 19(A)) are patterned by YBCO film on an MgO substrate 1a having a thickness t of 0.5 mm, and the filter is sealed in an aluminum-alloy package 1d. The package 1d and upper cover 1e (see FIG. 19(B)) prevents leakage of electromagnetic field, thereby uniformly cooling substrate 1a. FIG. 19(A) is a plan view in which an upper cover 1e of the package has been removed, and FIG. 19(B) is a sectional view taken along line AA in FIG. 19(A). Further, reference characters 1f, 1g represent coaxial connectors and 1h (see FIG. 19(B)) a ground formed by YBCO film having a thickness of 0.4 μm.
The electrical connections in the vacuum vessel are as shown in FIG. 20. For example, two channels of wireless receiving amplifiers are formed. Superconducting filters 1, 1′ exhibit a prescribed pass-band characteristic if they are cooled to a cryogenic temperature of 70 K, and output pass-band components from among signals contained in receive signals that enter from input terminals 7b, 7b′. Low-noise amplifiers (LNA) 2, 2′ amplify the signals that have passed through the superconducting filters 1, 1′, and the amplified signals are delivered from output terminals 8b, 8b′. The low-noise amplifiers 2, 2′ have a gain characteristic and a noise figure characteristic shown in FIGS. 21(A) and 21(B). The solid line indicates the characteristic at ordinary temperature (=23° C.) and the dashed line the characteristic at a cryogenic temperature of 77 K. It will be understood that when a cryogenic temperature is attained, the gain rises by 2 dB and the noise figure declines. That is, it is preferred that the low-noise amplifiers 2, 2′ be used at cryogenic temperatures rather than at ordinary temperature.
Thus, the superconducting filter 1 is accommodated within the vacuum vessel 3 and operates upon being cooled to cryogenic temperature of, e.g., T=70 K by the refrigerator 5. Further, if the low-noise amplifier (LNA) 2 that amplifies the received signal to a prescribed level also is cooled to a cryogenic temperature, then the noise figure can be reduced. In general, therefore, the low-noise amplifier is cooled at the same time as the superconducting filter 1. A signal received by an antenna (not shown) is input to the case 6 from the input terminal 7a via an antenna feeder, the signal propagates through the coaxial cables 9a, 9c, only a signal of the necessary frequency band is extracted by the superconducting filter 1, this signal is amplified to a prescribed signal level by the low-noise amplifier 2, and the resultant signal is output from the output terminal 8a. 
In a mobile communications system, the wireless receiving amplifier shown in FIG. 18 is installed outdoors, namely on the roof of a building, and hence is placed in a hostile environment of high temperatures and humidity as occur in mid-summer, etc. While thus exposed to very harsh conditions, the wireless receiving amplifier is required to exhibit stable operating reliability for an extended period of time of, e.g., tens of thousands of hours. However, since many sliding parts are used in the refrigerator 5, mechanical malfunction is a possibility. If the refrigerator 5 malfunctions, the temperature, which is being held at, e.g., T=70 K, naturally will rise and the superconducting filter 1 will no longer perform its original function. The result is communication failure. Accordingly, if the refrigerator develops a failure, it is necessary to have a function for detecting the failure immediately and reporting the failure, or a function for detecting the failure while it is still minor and reporting the same. In the prior art, there are arrangements in which refrigerator abnormality is detected by measuring the temperature in the vacuum vessel and performing monitoring to determine whether the temperature has exceeded a set temperature. However, the apparatus involved is large in size and of great weight. This does not conform to the requirement for an apparatus of smaller size and lighter weight.